1. Field of the Invention
This invention relates to seals which are used with movable members such as piston rods in shock absorbers, and more particularly to a method and apparatus for sealing a movable member with respect to a fixed member.
2. Description of Related Art
Shock absorbers are used in connection with automotive suspension systems to absorb unwanted vibrations which occur during driving. To absorb unwanted vibrations, shock absorbers are generally connected between the body and the suspension of the automobile. A piston is located within the shock absorber and is connected to the body of the automobile through a piston rod. Because the piston is able to limit the flow of damping fluid within the working chamber of the shock absorber when the shock absorber is compressed, the shock absorber is able to produce a damping force which counteracts the vibration which would otherwise be transmitted from the suspension to the body. The greater the degree to which the flow of damping fluid within the working chamber is restricted by the piston, the greater the damping forces which are generated by the shock absorber.
Shock absorbers typically have a seal disposed between the upper end cap of the shock absorber and the piston rod. The seal is used to prevent damping fluid in the working chamber from leaking to the outside of the shock absorber when the piston rod is displaced. Such seals typically have a plurality of annular lips which engage the piston rod to prevent damping fluid from flowing therebetween. Because the piston rod carries damping fluid from the working chamber to the seal upon sufficient displacement of the piston rod, the seal would often become lubricated after the automobile to which it was attached encountered a sufficiently rough surface.
While such seals were effective in preventing leakage of damping fluid from the working chamber to the outside of the shock absorber, the seals often had several disadvantages. After the shock absorber was stationary for a period of time, the damping fluid which coated the piston rod during movement would often drain into the reservoir of damping fluid in the working chamber leaving the piston rod uncoated. When the piston rod was then initially moved from its stationary position, a relatively large force was required to overcome the friction between the seal and the uncoated piston rod. In addition, the shock absorber would often produce a relatively loud noise or "grunt" when the piston rod was initially displaced after being held stationary for a period of time. This noise or "grunt" was also due to the relatively large frictional forces between the uncoated piston rod and the seal.